It is the main form, it is the one that appears when I run the program, the other forms and dialogs appear at the center of the screen and they have propertie of the border changed, I don't know why this is not appearing at the center.
The FormBorderStyle, MinimizeBox, and MaximizeBox properties allow you to control whether the form can be minimized, maximized, or resized at run time.
The FormBorderStyle property has an affect on size and location of your form. Sizable is the default setting. Here is a link to the documentation for the property, it may give some clues as to why the form behaves this way when setting this property.